1. Field of the Invention
The present invention relates to an electroluminescence (EL) display device, and more particularly, to an EL display device with improved external light coupling efficiency and brightness that may be easily manufactured.
2. Discussion of the Background
The external light coupling efficiency, ηex, of an EL display device is given by the following equation:ηex=ηin·ηout where, ηin and ηout denote the internal quantum coupling efficiency and the output quantum coupling efficiency, respectively. The internal quantum coupling efficiency, ηin, is determined by self-elimination within each layer. The output quantum coupling efficiency, ηout, is determined by the prevention of light to the outside due to the total internal reflection in each layer (i.e., failure to transmit the light to the outside due to the total internal reflection occurring at an interface because an incident angle of the light is greater than a critical angle of the light when the light is incident from a medium having a higher refractive index as compared to another medium having a lower refractive index). In the EL display device, a light emitting layer transmits the light through many layers before emitting the light to the outside, where variations in the refractive indexes of the many layers prevent emission of some light.
The output quantum coupling efficiency, ηout, or the light transmittance efficiency which depends on the total internal reflection at interfaces between the layers when the emissive layer emits light to the outside, may be expressed as:
                              η          out                =                              1            2                    ⁢                                    (                                                N                  out                                                  N                  in                                            )                        2                                              (        2        )            where Nout represents the refractive index of the emissive layer emitting the light, and Nin represents the refractive index of the emissive layer receiving the light. For example, the output light coupling efficiency of light transmitting through a layer with a refractive index of about 1.5 to a layer with a refractive index of about 1.2 is determined to be 32% using Equation 2, or about 70% of the light which enters the interface is not emitted to the outside.
There have been many efforts to prevent lowered external light coupling efficiency.
Japanese Patent Publication No. 4-192290 discloses an inorganic EL device where a plurality of condensing microlenses is the same size or larger than an inorganic EL element. The plurality of condensing microlenses is formed on an external surface of a transparent substrate on which the inorganic EL element is formed. Light incident to the interface between the transparent substrate and air at an angle greater than the critical angle has an incident angle less than the critical angle on the microlenses, thereby reducing the total internal reflection. Additionally, the light is emitted in a predetermined direction to improve brightness. However, the cited invention discloses the EL element is a surface light source; thus, diffused light, which is not focused, inevitably occurs when using a microlens that is the same size or larger than the EL element. Clarity of an image is reduced due to overlapping images produced by the adjacent EL elements.
Japanese Patent Publication No. 7-037688 discloses an EL element formed on a substrate having a cylindrically-shaped high refraction factor section composed of a material having a higher refraction index than the surrounding material formed around the side surface of the substrate. Light produced by the EL element is emitted through the high-refraction factor section to increase external light coupling efficiency. However, in the cited invention, the light transmitted through the high-refraction factor section is diffused light, as illustrated in FIG. 1 of the cited invention. Thus, the brightness of the light emitted to the front is not improved.
Japanese Patent Publication No. 10-172756 discloses an organic EL light emitting device having a plurality of condensing lenses formed between a lower electrode, composing the organic EL light emitting device, and a transparent substrate. The organic EL light emitting device and the condensing lens are disposed in correspondence with one another. Light transmitted through the EL light emitting device is incident to an interface of the transparent substrate at a smaller angle than the critical angle for increasing the external light coupling efficiency. However, in the cited invention, overlapping images produced by adjacent EL light emitting devices reduce image clarity.